In general, a radio apparatus for transmitting a high-speed digital signal, a broadband analog signal, or the like is composed of a transmitter having a function of producing a product of an intermediate-frequency (IF) band signal and a local oscillation (LO) signal and performing up-conversion so as to generate and transmit a radio-frequency (RF) modulated signal, and a receiver having a function of receiving the RF modulated signal, obtaining a product of the RF modulated signal and an LO signal, and performing down-conversion so as to generate an IF signal. In such a case, in order to maintain the quality of the transmitted signal, the IF signal input to the transmitter and the IF signal generated in the receiver must have a predetermined frequency difference therebetween, and variation in the phase difference with time is required to be small. Therefore, the local oscillators which generate LO signals in the transmitter and the receiver must be excellent in frequency stability and must be low in phase noise. In particular, in ranges of microwaves and millimeter waves of high frequencies, a dielectric resonator or a PLL (phase lock loop) circuit is employed so as to improve stability and reduce noise.
However, as the frequency to be used increases (to, e.g., a millimeter band of 30 GHz or higher), realization of an oscillator having high stability and low noise becomes difficult, and production cost increases. For example, in the case where a dielectric resonator is used, the Q value (quality factor) of the dielectric resonator decreases, and a desired performance cannot be attained. In the case where a PLL circuit is used, formation of a frequency divider in particular becomes difficult, among other problems. There exists a method in which an LO signal is obtained through frequency multiplication of a signal from a low-frequency oscillator. However, in general, this method requires an amplifier for increasing signal strength, which results in increased cost, increased size, and increased power consumption.
In order to solve these problems, there has been proposed a radio communication apparatus (self-heterodyne scheme) shown in FIG. 9 (described in Japanese Patent Application Laid-Open (kokai) No. 2001-53640). In this example, an IF modulated signal of data input to a transmitter 81 is multiplied at a mixer 83 by a local oscillation (LO) signal from a local oscillator 85, and unnecessary components are removed by a band-pass filter 86 so as to generate a radio-frequency (RF) modulated signal. In a power mixer 87, a portion of the LO signal is added to the RF modulated signal. The resultant radio signal is amplified to a higher signal level by means of an amplifier 88, and then transmitted from an antenna Tx. Meanwhile, in a receiver 82, the radio signal received by means of an antenna Rx is amplified to a higher signal level by means of an amplifier 91, passed through a filter 92 within the receiver, and demodulated into an IF signal at a squaring unit 93. In this method, an LO signal which is the same as that used for generation of the RF signal is transmitted as a radio signal. Accordingly, this method is advantageous in that influence of phase noise of the local oscillator 85, serving as an LO signal source, is canceled at the time of demodulation, and the IF signal obtained through demodulation has the same frequency as that of the original IF signal input to the transmitter.
Further, Japanese Patent Application Laid-Open (kokai) No. 2002-246921 discloses a transmission circuit in which an transmission IF modulated signal and an un-modulated carrier whose frequency is separated from the modulated signal by a frequency interval corresponding to a proper frequency of an IF signal obtained through demodulation at a receiver side are mixedly up-converted to a millimeter-wave band by use of a millimeter-wave band local oscillation signal.